I went to Sorrento in 2008. I flew to Naples airport and got a very good deal on a hire car from the airport. There is a large compound about 10 minute walk or a short bus ride from the airport terminal. There are several operators in the compound. My car was from Sixt, but Hertz, National, Avis etc were there.
I used
http://www.holidayautos.co.uk/ which is part of lastminute.com
Looking at a 7 day hire next April, you can get a 9 seater VW Caravelle for £545 from the airport. If you want it from downtown Sorrento the fee is £1600